Peripheral Blood Film Examination

Overview

A peripheral blood film (blood smear) is a thin layer of blood spread on a glass slide, stained, and examined under a microscope. Blood-film examination provides morphological information about red cells, white cells, and platelets that automated cell counters may not fully capture, and remains an important complementary technique even in highly automated hematology laboratories.

Why blood films are examined

Common reasons for blood-film review include:

• Instrument-generated flags suggesting an abnormal cell population
• Unexplained or unexpected changes in CBC parameters
• Clinical request for morphological assessment
• Confirmation of automated results, such as a low platelet count that may reflect clumping
• Investigation of suspected hematologic disorders
• New patient results outside a defined critical or panic range, according to laboratory policy

Laboratories generally define criteria for when a blood film is reviewed, based on instrument flags, result criteria, delta checks, or clinician request, often summarized in a documented blood-film review policy.

Film preparation

A well-made film has a gradual transition from a thick to a thin area — sometimes described as having a head, body, and thin "feathered" tail — without holes, streaks, or an abrupt straight edge, and is dried and stained according to the laboratory's validated procedure (commonly a Romanowsky-type stain such as Wright or Wright-Giemsa). The feathered edge and monolayer region (where red cells are evenly spaced, not overlapping) are typically the areas examined for morphology and differential counting.

Systematic examination approach

A systematic approach to film examination commonly includes:

1. Low-power (scanning) review of the film edges and feathered tail for platelet clumps, large or abnormal cells, and overall cellularity.
2. Assessment of red-cell, white-cell, and platelet distribution and estimated numbers relative to the field.
3. High-power examination of the monolayer area for detailed red-cell morphology, white-cell differential and morphology, and platelet morphology.
4. Documentation of findings using standardized terminology and, where applicable, semi-quantitative grading.

What is assessed on a blood film

Red blood cells

• Size variation (anisocytosis)
• Shape variation (poikilocytosis) — for example, target cells, spherocytes, schistocytes, sickle cells, elliptocytes, or teardrop cells (dacrocytes)
• Color/hemoglobinization (e.g., hypochromia, polychromasia)
• Inclusions, where present (for example, Howell-Jolly bodies or basophilic stippling)
• Arrangement, such as rouleaux formation or agglutination

White blood cells

• Differential count confirmation
• Morphological features of individual cell lines, such as toxic granulation or left shift in neutrophils
• Presence of immature or atypical cells, such as blasts or atypical lymphocytes
• Nuclear and cytoplasmic features relevant to specific conditions

Platelets

• Estimated platelet number relative to red cells
• Platelet size and clumping
• Giant platelets, where present

Reporting

Blood-film findings are typically reported using standardized descriptive terminology and, where relevant, semi-quantitative grading (for example, 1+ to 4+ for a given morphological feature), according to the laboratory's procedure. Findings are considered alongside the CBC, clinical information, and other laboratory results rather than interpreted in isolation.

Quality considerations

Reliable blood-film examination depends on a well-prepared and well-stained slide, systematic examination technique, staff competency, and, where applicable, correlation with automated results and internal quality-assurance processes such as slide review, image-based verification, or digital morphology systems used to support and standardize interpretation.

Key points

• Blood-film examination adds morphological information beyond automated cell counts.
• Films are reviewed based on defined laboratory criteria, including instrument flags and delta checks.
• A systematic low-to-high-power examination approach helps ensure consistent findings.
• Red cells, white cells, and platelets are each assessed systematically.
• Reporting should use standardized terminology and grading where applicable.
• Blood-film findings are interpreted together with other clinical and laboratory information.

References

• NCBI. MeSH: Blood Smear.
• MedlinePlus. Blood Smear.
• ISO 15189:2022. Medical laboratories — Requirements for quality and competence.